Electron-beam cured emulsion pressure-sensitive adhesives

ABSTRACT

Electron-beam cure by itself or in combination with multifunctional additives is used to improve the high-temperature shear properties of emulsion polymerized pressure-sensitive adhesives.

This is a continuation of application Ser. No. 07/292,654 filed Dec. 28,1988, now U.S. Pat. No. 5,011,867, issued Apr. 30, 1991.

BACKGROUND OF THE INVENTION

The present invention is directed to electron beam or EB curing ofemulsion-based pressure-sensitive adhesives to provide apressure-sensitive adhesive with superior elevated temperature shearproperties.

Concern over the environment has made attractive pressure-sensitiveadhesive polymers formed by emulsion polymerization, as opposed topolymerization in organic solvents. Emulsion polymers can be coated ontoa substrate, e.g. face material, release liner, and/or a transfersurface, and a pressure-sensitive adhesive layer formed upon waterevaporation.

The possibility of preparing a pressure-sensitive adhesive in anemulsion and UV curing of pressure-sensitive adhesives has beenmentioned in the art. Examples include U.S. Pat. No. 3,637,615 toCoffman, U.S. Pat. No. 4,069,123 to Skoultchi et al and U.S. Pat. No.4,185,050 to Lazear et al. This art has however only illustrated thepreparation of polymers in an organic solvent and not in an aqueousmedia and therefore the effect of UV curing on a polymer prepared in anaqueous media has not been demonstrated.

We have found emulsion-based pressure-sensitive adhesives to have poorhigh-temperature peel and shear properties.

It is the purpose of the instant invention to provide a technique toimprove the high-temperature performance of emulsion-basedpressure-sensitive polymers without sacrificing significantly theambient temperature performance.

SUMMARY OF THE INVENTION

There are provided emulsion pressure-sensitive adhesive compositionscurable on exposure to electron-beam (EB) radiation to form adhesiveshaving increased elevated temperature shear properties without asignificant change in ambient temperature properties. The adhesives areformed of one or more polymers having a glass transition temperature ofat least 20° C. below the use temperature. They may be branched emulsionpolymers polymerized at temperatures of at least about 30° C., andpredominantly linear emulsion polymers polymerized at a temperature lessthan about 20° C., and mixtures thereof.

The emulsion polymers are independently formed in emulsion of at leastone first monomer which, when homopolymerized, has a glass transitiontemperature less than about 0° C. and at least one second monomer which,when homopolymerized, has a glass transition temperature greater thanabout 0° C.

A multifunctional additive is provided to aid cure of branched polymersand benefits all emulsion systems. Multifunctional additives arepreferably present in an amount up to about 10% by weight based on theweight of the emulsion-polymers. If employed, the multifunctionaladditive is preferably present in a concentration of from about 0.2 toabout 5% by weight of the emulsion polymer while presently preferredmultifunctional additives consist of pentaerythritol triacrylate,trimethylolpropane trimethacrylate, and 1,6-hexanediol diacrylate, andmixtures thereof.

Cure is achieved at EB dosages up to about 100 kiloGray (kGy) preferablyfrom about 5 to about 50 kGy. There may be realized upon EB curepressure-sensitive adhesives having 180° peel in excess of about 250 N/mand a loop tack in excess of about 200 N/m while achieving a 70° C.shear of at least 10 kiloseconds normally in excess of 86 kiloseconds.Shear values at 100° C. in excess of 10 kiloseconds and surprisingly inexcess of 86 kiloseconds can also be achieved. As compared to uncuredpolymers elevated temperature shear (70° C. and 100° C.) may beincreased several fold.

Acrylic-based emulsion polymers are preferred. The preferred polymershave a storage modulus (G') of from about 3.2×10⁴ Pa to about 1.2×10⁵ Paat a frequency of 10⁻¹ s⁻¹ and from about 1.4×10⁵ Pa to about 5.2×10⁵ Paat a frequency of 10² s⁻¹ and a loss modulus (G") of from about 9.5×10³Pa to about 3.1×10⁴ Pa at a frequency of 10⁻¹ s⁻¹ and about 7×10⁴ Pa toabout 5.6×10⁵ Pa at a frequency of 10² s⁻¹. Acrylic emulsion polymersare preferably formed of about 50 to about 95% by weight of a firstmonomer which when homopolymerized has a glass transition temperatureless than about -25° C. and at least one second monomer which whenhomopolymerized has a glass transition temperature greater than about-25° C.

THE DRAWING

The attached drawing shows the change in adhesion bond with time of thesame composition which had not been exposed (Control L) to electron-beamradiation and which had been exposed to such radiation showing that EBradiation does not detract from the ability of an adhesive to increaseadhesion with dwell.

DETAILED DESCRIPTION

According to the present invention, high-temperature performance ofemulsion pressure-sensitive adhesives, preferably acrylic-basedpressure-sensitive adhesives, is significantly improved by the use ofelectron-beam radiation. In one embodiment, the emulsionpressure-sensitive adhesives are prepared in a conventional manner andmodified by the addition of multifunctional additives and after coatingonto a suitable substrate, electron-beam (EB) cured at dosage levelssufficient to increase elevated temperature adhesive performance. Dosagelevels employed are up to about 100 kiloGray (kGy) preferably from about5 to about 50 kGy. For instance, dosages as low as 10 kiloGray (kGy) canenable a realization of and increase the 70° C. shear from 0.4kilosecond to an excess of 86 kiloseconds at a minimal loss of ambienttemperature properties such as 180° peel and loop tack.

By the term "multifunctional additive" there is meant monomers, dimers,trimers, etc., having two or more reactive sites which undergo reactionswith the pressure-sensitive adhesive on exposure to electron-beam.

The amount of multifunctional additive can be significantly minimized oreliminated if there is utilized, as part of the pressure-sensitiveadhesive, emulsion polymers synthesized at temperatures below about 25°C. to provide polymers of greater linearity and of relatively smallparticle size. Such pressure-sensitive adhesives, when subjected toelectron-beam radiation at levels of about 30 kGy or more, exhibitunusually high elevated temperature shear properties again exceeding 86kiloseconds at temperatures of 70° C. and 100° C.

The use of electron-beam radiation enables effective resort towater-based systems for producing high performance pressure-sensitiveadhesives.

The pressure-sensitive adhesives preferably provided in accordance withthe instant invention have a glass transition temperature at least about20° C. to about 70° C. below use temperature and rheology whereinstorage modulus (G') and loss modulus (G") are respectively in the rangeof about:

    ______________________________________                                        G'                                                                            3.2 × 10.sup.4 Pa to 1.2 × 10.sup.5 Pa (at a frequency of         10.sup.-1 s.sup.-1)                                                            and 1.4 × 10.sup.5 Pa to 5.2 × 10.sup.5 Pa (at a frequency       of 10.sup.2 s.sup.-1)                                                         G9.5 × 10.sup.3 Pa to 3.1 × 10.sup.4 Pa (at a frequency of        10.sup.-1 s.sup.-1)                                                            and 7 × 10.sup.4 Pa to 5.6 × 10.sup.5 Pa (at a frequency of      10.sup.2 s.sup.-1)                                                            ______________________________________                                    

as determined by Method E of ASTM D-4065-82 modified for shearoscillation

High-temperature performance pressure-sensitive adhesives are producedin accordance with the instant invention by coating emulsionpressure-sensitive adhesive polymers onto a substrate, evaporating thewater and subjecting the dry coatings to electron-beam radiation atlevels sufficient to increase high-temperature performance with minimalloss of other properties.

Emulsion-based pressure-sensitive adhesive polymers for use in theinvention may be prepared in accordance with the present invention bycopolymerizing monomers to yield a polymer having a glass transitiontemperature from about 20° C. to about 70° C. below use temperaturepreferably from about 50° C. to about 70° C. below use temperature. Toprepare an emulsion having direct utility for substrate coating, solidscontent will generally range from about 40 to about 60% by weightsolids. Catalysts such as tertiary butyl hydroperoxide or the like maybe present in an amount of from about 0.15 to about 0.5 part by weightper 100 parts monomers with surfactant levels ranging from about 0.5% toabout 5% by weight based on weight of the monomers.

Reaction temperature, catalyst content and surfactant levels control thenature of the polymer formed. While not bound by theory, elevatedreaction temperatures, e.g. temperatures of about 30° C. or more, tendto yield polymers which are more branched than polymers synthesized atlower temperatures. Thus, as used herein, the term "branched polymer"means a polymer emulsion synthesized at a temperature above about 30° C.and is not normally responsive to the action of EB-radiation in theabsence of a multi-functional monomer; while the term "linear polymer"means a polymer emulsion synthesized at a temperature below about 25° C.and responsive to the action of EB radiation without the addition of amultifunctional additive. An exception for the "branched polymers" maybe situations where a pendant group(s) of the monomers leading to thepolymer are reactive and/or where the surfactant employed is reactivewith the monomers and/or polymer. Reactive surfactants are those basedon C₇ -C₁₆ unsaturated fatty acids. The inclusion of a multifunctionaladditive, generally in levels in an amount up to about 10% preferablyfrom about 0.2 to about 5% by weight of total adhesive polymer content,enables a marked increase of high-temperature performance at EB dosagelevels as low as 10 kGy or less. In fact, there appears to be a peakwhere, after some level of EB dosage is reached, high-temperatureperformance although improved, begins to drop off due to excessivecrosslinking.

By a "multifunctional additive" as used herein, there is meant anadditive having 2 or more pendant functional groups, preferably acrylateor methacrylate groups. As representative multifunctional additivesthere may be mentioned pentaerythritol triacrylate (PETA),triethyleneglycol diacrylate, triethyleneglycol dimethacrylate,trimethylolpropane trimethacrylate (TMPTMA), trimethylolpropanetriacrylate, tripropyleneglycol diacrylate, tripropyleneglycoldimethacrylate, 1,3-butyleneglycol dimethacrylate, tetraethyleneglycoldimethacrylate, 1,6-hexanediol diacrylate (HDDA), and the like.Multifunctional additives having ethoxylated and propoxylated backbonesmay also be effectively used. As indicated, concentrations will varydepending on polymer nature and electron-beam dosage levels.

In general, multifunctional additive concentrations can range from 0 toabout 10% or more by weight of the emulsion polymer. None is requiredfor highly linear polymers. Their presence is however beneficial. Blendsof linear and branched polymers need multifunctional additivescorresponding to the level of branched polymer in the mixture. Apresently preferred range is from about 0.2 to about 5% by weight basedon the weight of the polymers.

While not bound by theory, a possible explanation for the beneficial useof multifunctional additives is that they may have a plasticizing effecton high gel content of branched emulsion polymers, i.e. polymersprepared at temperatures of about 30° C. or more. Because of high gelcontent, diffusion of polymer chains between neighboring particles maybe restricted. The addition of the multifunctional additives may have aplasticizing effect and lead to improved diffusion of the polymer chainsbetween particles, and crosslinking the polymer chains by EB radiationprovides high elevated temperature shear with minimal, if no, loss inambient temperature performance. Another possibility is that bridgingbetween polymer particles occurs where otherwise crosslinking would berestricted to the body of the particle with interparticle bondsremaining low.

Forming the emulsion using low temperatures, high surfactant levels, andlow initiator levels forms a pressure-sensitive adhesive of relativelylow particle size. These pressure-sensitive adhesives, when alsosubjected to relatively low levels of EB radiation, e.g. about 30 kGy,also exhibit unusually high elevated temperature performance despite theabsence of added multifunctional additives, although the multifunctionaladditives can be effectively used to lower EB dosage levels.

A possible explanation for this is that at low temperatures, branchingand crosslinking reactions are minimized and the polymer obtained is oflower gel content and is more linear than those synthesized at highertemperatures. This appears to enhance interparticle diffusion of thepolymer chains despite even higher molecular weight. The high degree ofdiffusion leads to greater entanglement between the polymer chains ofneighboring particles allowing the use of EB radiation without amultifunctional additive to achieve high elevated temperatureperformance.

The emulsion polymers provided as pressure-sensitive adhesives of theinstant invention generally comprise from about 50% to about 95% byweight of one or more first monomer, which if homopolymerized, wouldhave a glass transition temperature of less than about -25° C. based onthe total weight of the monomers. Exemplary are the alkyl acrylates,such as butyl acrylate, propylacrylate, 2-ethylhexyl acrylate, isooctylacrylate, isodecyl acrylate; mono and diesters of unsaturateddicarboxylic acids, such as dibutyl fumarate and the like.

The balance of the monomer system is comprised of second monomers whichif homopolymerized would have a glass-transition temperature greaterthan -25° C., normally greater than about 10° C. Among such monomersthere may be mentioned ethyl acrylate, alkyl methacrylates, such asmethyl methacrylate, ethyl methacrylate, butyl methacrylate and thelike; copolymerizable vinyl-unsaturated monomers, such as vinyl acetate,vinyl propionate and the like; and styrenic monomers, such as styrene,methylstyrene and the like; unsaturated carboxylic acids, such asacrylic acid, methacrylic acid, itaconic acid, fumaric acid and thelike.

As indicated, emulsion polymers prepared in accordance with the presentinvention, to be functional pressure-sensitive adhesives, must have anet effective glass-transition temperature of from about 20° to about70° C. below the use temperature, preferably from about 50° to about 70°C. below the use temperature. Accordingly, the monomers used in thepreparation of polymers of the instant invention are proportioned toprovide polymerization products falling within the designatedglass-transition temperature. It is for this reason that the firstmonomers, preferably one or more alkyl acrylates, are provided as thebulk of the monomers employed and are typically present in an amount offrom about 50 to about 95 percent by weight based on the total weight ofthe monomers. It is also desired that they have pendant acrylate ormethacrylate groups to enhance EB cure.

The emulsions formed including added multifunctional additives arecoated on any suitable substrate such as face stock, release liner stockor transfer surfaces by means known in the art. Net solids coatingweights generally range from about 40 or less to about 60 grams or moreper square meter. Following evaporation of water, the residual solidsare exposed to electron-beam radiation at levels sufficient to increasehigh-temperature properties, particularly shear, without adverselyaffecting peel and tack at normal use temperatures. Electron-beamdosages may range from about 10 kiloGray (kGy) or less up to about 100kGy preferably 50 kGy or less depending on the nature of the polymer andamount of multifunctional additives present, with required dosages beinglowered by the presence of a multifunctional additive. The presence of amultifunctional additive can also create a limit on the EB dosages used.A peak is reached at some level after which the level of increase ofelevated temperature shear will be reduced but still be above the levelwhich existed prior to cure.

It will be appreciated that there may be included in thepressure-sensitive adhesives, tackifiers, fillers, plasticizers and thelike.

The following Examples illustrate the invention. With reference thereto,multifunctional monomer is given in percent by weight. Test methodsutilized were as follows: shear by ASTM D-3654-78 and D-1000-68 and PSTC(PressureSensitive Tape Council) Test #7, 6th Edition (1/2"×1/2" 500 G,0 dwell at 70° C. and/or 100° C.); 180 peel by PSTC Test #1, 6th Edition(20 minute dwell) and loop tack by PSTC Test #7, 6th Edition.

Emulsion Polymer 1 (E-1) and Controls A to E and Examples 1-6

To 1-liter jacketed reactor flask, fitted with a mechanical stirrer,there was charged with 85.0 grams deionized water, 0.05 gram ferric EDTA(ethylenediamine- tetraacetic acid), 0.5 gram diammonium phosphate, 0.55gram DoWfax 2Al (commercially available from Dow Chemical Co., 45%active) and 50.0 grams of dibutyl fumarate.

A monomer pre-emulsion was made up by dissolving 8.93 grams of TritonX-200 (commercially available from Rohm & Haas Co., 28% active) in 75.0grams deionized water and adding a mixture of 137.5 grams 2-ethyl hexylacrylate, 57.5 grams methyl methacrylate, 5.0 grams acrylic acid, 1.0gram t-butyl hydroperoxide (70% active), 0.35 gram n-dodecyl mercaptan,and 0.375 gram 1,6-hexanediol diacrylate. 57.13 grams of thispre-emulsion was added to the reactor flask, the contents purged with N₂and temperature maintained at 30° C.

An aqueous solution was made up by dissolving 1.0 gram of sodiumformaldehyde sulfoxylate (commercially available from Diamond Shamrockunder the trade name Hydrosulfite AWC) in 75 grams deionized water. 11.4grams of this solution was added to the reactor flask and 10 minuteslater the contents were heated to 45° C. and the simultaneous additionof the remaining 64.6 grams of aqueous solution and 228.53 grams ofpre-emulsion was started. The pre-emulsion was added over 2.5 hours andaqueous solution, over 3 hours. The temperature was maintained between45 and 48° C. throughout the addition. After the additions werecomplete, the contents were held at this temperature for an additionalhour and then discharged. The total coagulum on filtering through 150micron screen was less than 0.02% (dry).

The performance properties of the adhesive coating obtained from theabove emulsion polymer (E-1) are listed in Table I. The latex had asolids content of 51.0%, viscosity at pH=7.0 being 0.44 Pa.S (BrookfieldRVT, #4 @ 100 rpm, NH₃ as the base), particle size of 139 nm andexcellent mechanial stability (Haake, 40,000 s⁻¹). In the evaluation,the multifunctional acrylate, if present, was added to the emulsion andthe emulsion coated onto 2 mil Mylar to a level up to 54 g/m² and if EBcured, EB cured through Mylar.

                                      TABLE I                                     __________________________________________________________________________                          180° Peel                                                                    Loop Tack                                                                           RT Shear                                                                              70° Shear                                      EB  RT, SS, 20'                                                                         RT, SS                                                                              1/2 kg, 1/2 × 1/2,                                                              1/2 kg, 1/2 × 1/2, SS         Ex/Cont.                                                                           Latex                                                                             MFA      Dosage                                                                            N/m   N/m   ksec    ksec                                __________________________________________________________________________    Cont. A                                                                            E-1 0        0   653   593   86.4+   0.4                                 Cont. B                                                                            E-1 0        10 kGy                                                                            607   527   85.5    1.1                                 Cont. C                                                                            E-1 0        20 kGy                                                                            620   487   84.8    1.1                                 Cont. D                                                                            E-1 2% HDDA  0   613   580    48.67+ 0.46                                Ex. 1                                                                              E-1 2% HDDA  10 kGy                                                                            575   530   86.4+   86.4+                               Ex. 2                                                                              E-1 2% HDDA  30 kGy                                                                            580   525   86.4+   47.9                                Ex. 3                                                                              E-1 2% TMPTMA                                                                              10 kGy                                                                            520   470   86.4+   86.4+                               Ex. 4                                                                              E-1 2% TMPTMA                                                                              30 kGy                                                                            480   445   86.4+   13.0                                Cont. E                                                                            E-1 2% PETA  0   640   570   86.4+   0.4                                 Ex. 5                                                                              E-1 2% PETA  10 kGy                                                                            470   385   86.4+   39.5                                Ex. 6                                                                              E-1 2% PETA  30 kGy                                                                            465   410   86.4+   17.4                                __________________________________________________________________________     SS = stainless steel                                                          + means samples did not fail at this point.                              

Emulsion Polymers (E-2) and (E-3) and Controls G and H and Examples 7-13

This emulsion polymer was synthesized using the equipment as outlined inExample 1. The reactor was charged with 175 grams deionized water, 0.05gram ferric EDTA and 1.88 grams Aerosol MA 80 (commercially availablefrom American Cyanamid, 80% active) A monomer pre-emulsion was made upby dissolving 20.68 grams of Triton X-200 and 7.0 grams of Aerosol MA-80in 116.8 grams of distilled water and adding a mixture of 266.2 grams2-ethylhexyl acrylate, 14.01 grams acrylic acid and 0.56 gram t-butylhydroperoxide. An aqueous solution was made up by dissolving 0.6 gram ofhydrosulfite AWC in 150 grams of deionized water. The reactor mixturewas purged with nitrogen, cooled to 5° C., and the pre-emulsion andaqueous solution feeds were started simultaneously. The pre-emulsion wasadded over 2.5 hours, aqueous solution over 2.75 hours, temperaturebeing maintained between 5° and 8° C. The contents were held at 5° C.for 2 more hours, discharged (after filtering through cheesecloth) andneutralized to a pH of 7.2 with NH₃. The total coagulum was 1.0 gram,the solids content of the emulsion was 38.9%, and the particle size was139 nm. The performance of this polymer and another polymer (E-3)prepared in the same manner are shown in Table II. E-3 contained 90parts 2-ethyl hexyl acrylate and 10 parts acrylic acid. Respectivecoating weights were 42 and 44 g/m² on 2 mil Mylar with cure throughMylar.

                                      TABLE II                                    __________________________________________________________________________                 180° Peel                                                                    Loop Tack                                                                           Polyken                                                                            RT Shear 70° Shear                                                                       100° C. Shear                   EB  RT, SS, 20'                                                                         RT, SS                                                                              A-1-1                                                                              SS, 1/2 × 1/2, 500 g                                                             SS, 1/2 × 1/2, 500                                                               SS, 1/2 × 1/2, 500                                                      g                             Ex/Cont.                                                                           Latex                                                                             Dosage                                                                            N/m   N/m   kPa  ksec     ksec     ksec                          __________________________________________________________________________    Cont. F                                                                            E-2 0   370   290   185  14.6     0.12     0.03                          Ex. 7                                                                              E-2 10 kGy                                                                            375   230   173  216+     25.5     0.2                           Ex. 8                                                                              E-2 30 kGy                                                                            355   250   170  216+     86.4+    86.4+                         Ex. 9                                                                              E-2 50 kGy                                                                            230   190   130  86.4+    86.4+    225                           Ex. 10                                                                             E-2 80 kGy                                                                            280   250   162  46+      86.4+    86.4+                         Cont. G                                                                            E-3 0   297   310   253  48.2     0.27     --                            Ex. 11                                                                             E-3 30 kGy                                                                            243   270   206  86.4+    86.4+    234+                          Ex. 12                                                                             E-3 50 kGy                                                                            140   243   206  86.4+    86.4+    234+                          Ex. 13                                                                             E-3 80 kGy                                                                            150   183   119  86.4+    86.4+    86.4+                         __________________________________________________________________________

Emulsion Polymers (E-4) and (E-5) and Controls I and J and Examples14-18

This emulsion polymer was synthesized using the equipment and procedureoutlined for polymer E-2. The initial reactor charge had in addition tothe ingredients listed in Example 2, 11.07 grams of Triton X-200. Thepre-emulsion was made up by dissolving 22.14 grams of Triton X-200 in125 grams of deionized water and adding a mixture of 217.5 grams of2-ethylhexyl acrylate, 67.5 grams of vinyl acetate, 15.0 grams ofacrylic acid, and 0.6 grams of t-butyl hydroperoxide. The aqueoussolution was made up by dissolving 3.75 grams of Aerosol MA-80 and 0.6gram of hydrosulfite in 150.0 grams of deionized water. The resultinglatex had very low viscosity and no coagulum. The solids content of thelatex was 38.0%, particle size=161 nm, and viscosity at pH=6.2 was 4.56Pa.s (Brookfield, RVT, #7 @ 100 rpm).

The performance of this polymer is listed in Table III (E-4) along witha polymer prepared in the same manner but containing 95 partsisooctylacrylate and 5 parts acrylic acid (E-5). The latexes were coatedto a level between 45 and 47 gr/m² on 2 mil Mylar with cure throughmylar.

The product of Example 15 when allowed to dwell for 24 hours displayedan increase from 180° peel to 490 N/m and in Example 16 increased to 455N/m.

                                      TABLE III                                   __________________________________________________________________________                 180° Peel                                                                    Loop Tack                                                                           Polyken                                                                            RT Shear 70° C. Shear                                                                    100° C. Shear                   EB  RT, SS, 20'                                                                         RT, SS                                                                              A-1-1                                                                              SS, 1/2 × 1/2, 500 g                                                             SS, 1/2 × 1/2, 500                                                               SS, 1/2 × 1/2, 500                                                      g                             Ex/Cont.                                                                           Latex                                                                             Dosage                                                                            N/m   N/m   kPa  ksec     ksec     ksec                          __________________________________________________________________________    Cont. I                                                                            E-4 0   297   383    61  86.4+    86.4+    0.12                          Ex. 14                                                                             E-4 30 kGy                                                                            283   383    69  86.4+    86.4+    86.4+                         Ex. 15                                                                             E-4 50 kGy                                                                            283   297    85  86.4+    86.4+    240+                          Cont. J                                                                            E-5 0   353   378   189  177      0.19     0                             Ex. 16                                                                             E-5 30 kGy                                                                            260   322   180  421.2+   86.4+    86.4+                         Ex. 17                                                                             E-5 50 kGy                                                                            218   290   185  421.2+   86.4+    86.4+                         Ex. 18                                                                             E-5 80 kGy                                                                            172   267   170  86.4+    86.4+    86.4+                         __________________________________________________________________________

Control K and Examples 19-26

Table IV shows the blend exhibiting high 180° peel and tack valuecompared to base low-temperature polymers retaining the elevatedtemperature (70° C.) shear properties. Table V shows the good peeladhesion properties to different substrates at elevated temperature. Forthis test the adhesive was coated on 2 mil Mylar at 99 g/sq. m. Peelvalues are in N/m for the substrates shown.

FIG. 1 illustrates the value of Composition B as a repositionableadhesive in that bond strength more than doubled after 3 days dwell at70° C. It is significant that electron-beam cure, while providing thebenefit of elevated temperature shear, did not detract from theadvantageous peel adhesion buildup on extended dwell.

                                      TABLE IV                                    __________________________________________________________________________                           180° Peel                                                                    Loop Tack                                                                           RT Shear 70° C.                                                                          100° C.                                                                Shear                                   EB  Dwell                                                                             RT, SS, 20'                                                                         RT, SS                                                                              SS, 1/2 × 1/2, 500                                                               SS, 1/2 × 1/2, 500                                                               SS, 1/2 × 1/2,                                                          500 g                    Ex/Cont.                                                                           Latex     Dosage                                                                            (Hr.)                                                                             N/m   N/m   ksec     ksec     ksec                     __________________________________________________________________________    Ex. 14                                                                             E-4       30 kGy  283   383   86.4+    86.4+    86.4+                    Ex. 1                                                                              E-1 + 2% HDDA                                                                           10 kGy                                                                            0   575   530   86.4+    86.4+    1.14                     Cont. K                                                                            A         0   0   470   470   86.4+    0.48     --                       Ex. 19                                                                             "         10 kGy                                                                            0   470   510   86.4+    86.4+    86.4+                    Ex. 20                                                                             "         30 kGy                                                                            0   430   490   86.4+    86.4+    86.4+                    Ex. 21                                                                             "         30 kGy                                                                            0     525(1)                                               Ex. 22                                                                             "         50 kGy                                                                            0   470   410   86.4+    86.4+    86.4+                    Ex. 23                                                                             "         50 kGy                                                                            24  540                                                    Ex. 24                                                                             B         10 kGy                                                                            0   450   450   86.4+    86.4+    86.4+                    Ex. 25                                                                             "         30 kGy                                                                            0   440   440   86.4+    86.4+    86.4+                    Ex. 26                                                                             "         30 kGy                                                                            24  580                                                    __________________________________________________________________________     A = 44E4/56E-1/0.57 HDDA                                                      B = 64E4/36E-1/0.36 HDDA                                                      (1) = 70° C. peel                                                 

                  TABLE V                                                         ______________________________________                                                  Substrate                                                           180° Peel                                                                        SS            ABS    NAD                                            ______________________________________                                         70° C.                                                                          780           480    455                                            100° C.                                                                          400           390    350                                            ______________________________________                                         ABS = AcrylonitriteButadiene-Styrene                                          NAD = Surfacecoated with a solventbased paint, i.e. nonaqueous dispersion

Table VI shows the glass transition temperature and mechanicalproperties of various emulsion polymers of this invention.

                  TABLE VI                                                        ______________________________________                                                EB                                                                            Dosage  Tg       Frequency                                            Latex   (kGy)   °C.                                                                             rad/sec.                                                                              G' Pa  G" Pa                                 ______________________________________                                        1)  E-2      0      -67.5  a)   10.sup.-1                                                                          3.4 × 10.sup.4                                                                 1.4 × 10.sup.4                                         b)   10.sup.2                                                                           1.4 × 10.sup.5                                                                 7.0 × 10.sup.4              2)  E-3      0      -68    a)   10.sup.-1                                                                          3.2 × 10.sup.4                                                                 1.3 × 10.sup.4                                         b)   10.sup.2                                                                           1.4 × 10.sup.5                                                                 7.1 × 10.sup.4              3)  E-4      0      -50    a)   10.sup.-1                                                                          1.0 × 10.sup.5                                                                 2.8 × 10.sup.4                                         b)   10.sup.2                                                                           4.0 × 10.sup.5                                                                 2.4 × 10.sup.5              4)  E-5      0      -53.6       --   --     --                                5)  E-2     50      --          --   --     --                                6)  E-4     50      --     a)   10.sup.-1                                                                          1.2 × 10.sup.5                                                                 3.1 × 10.sup.4                                         b)   10.sup.2                                                                           4.8 × 10.sup.5                                                                 3.3 × 10.sup.5              7)  E-5     50      --     a)   10.sup.-1                                                                          3.6 × 10.sup.4                                                                 9.5 × 10.sup.3                                         b)   10.sup.2                                                                           1.4 × 10.sup.5                                                                 7.2 × 10.sup.4              8)  Ex. 25  30      --     a)   10.sup.-1                                                                          7.4 × 10.sup.4                                                                 2.3 × 10.sup.4                                         b)   10.sup.2                                                                           5.2 × 10.sup.5                                                                 3.7 × 10.sup.5              ______________________________________                                    

What is claimed is:
 1. A cured pressure-sensitive adhesive whichcomprises a branched chain emulsion polymer having a glass transitiontemperature of at least 20° C. below the use temperature, and formed byemulsion polymerization of at least one first monomer which, whenhomopolymerized, has a glass transition temperature less than -25° C.and at least one second monomer which, when homopolymerized, has a glasstransition temperature greater than -25° C., and said emulsion polymerbeing cured in the presence of from about 0.2 to about 10% by weightbased on the weight of the emulsion-polymer of a multifunctionaladditive and exposure to electron beam radiation at a dosage of up to100 kGy and sufficient to provide a 70° C. shear of at least 10kiloseconds and a 180° peel of at least about 250 N/m, and a loop tackof at least about 200 N/m at a coat weight of from about 40 to 60 g/m².2. A cured pressure-sensitive adhesive as claimed in claim 1 in whichthe multifunctional additive is present in a concentration of from about0.2 to about 5% by weight of the emulsion polymer.
 3. A curedpressure-sensitive adhesive as claimed in claim 1 in which themultifunctional additive is selected from the group consisting ofpentaerythritol triacrylate, trimethylolpropane trimethacrylate, and1,6-hexanediol diacrylate, and mixtures thereof.
 4. A curedpressure-sensitive adhesive as claimed in claim 1 in which the firstmonomer is selected from the group consisting of alkyl acrylates,monoesters of unsaturated dicarboxylic acids and diesters of unsaturateddicarboxylic acids, and mixtures thereof.
 5. A cured pressure-sensitiveadhesive as claimed in claim 4 in which the second monomer includes anunsaturated carboxylic acid.
 6. A cured pressure-sensitive adhesive asclaimed in claim 4 in which the multifunctional additive is selectedfrom the group consisting of pentaerythritol triacrylate,trimethylolpropane trimethacrylate, and 1,6-hexandiol diacrylate, andmixtures thereof.
 7. A cured pressure-sensitive adhesive as claimed inclaim 1 in which the electron-beam dosage was from about 5 to about 50kGy.
 8. A cured pressure-sensitive adhesive which comprises a curedbranched chain acrylic emulsion polymer having a glass transitiontemperature of 20° C. to about 70° C. below the use temperature, saidbranched chain acrylic emulsion polymer polymerized in emulsion attemperatures of at least about 30° C., said emulsion polymer formed ofat least one first monomer which, when homopolymerized, has a glasstransition temperature less than -25° C. and at least one second monomerwhich, when homopolymerized, has a glass transition temperature greaterthan -25° C., said emulsion polymer being cured in the presence of about0.2 to about 10% by weight of a multifunctional additive based on theweight of the emulsionpolymer, the emulsion-polymer includes a, saidpressure-sensitive adhesive being cured by exposure to electron beamradiation at a dosage up to 100 Gy to provide a pressure-sensitiveadhesive having a 70° C. shear of at least 86 kiloseconds, a 180° peelin excess of about 250 N/m, and a loop tack in excess of about 200 N/mat a coat weight of 40 to 60 g/m².
 9. A cured pressure-sensitiveadhesive as claimed in claim 8 in which the multifunctional additive ispresent in a concentration of from about 0.2 to about 5% by weight ofthe emulsion polymer.
 10. A cured pressure-sensitive adhesive as claimedin claim 8 in which the multifunctional additive is selected from thegroup consisting of pentaerythritol triacrylate, trimethylolpropanetrimethacrylate, and 1,6-hexanediol diacrylate, and mixtures thereof.11. A cured pressure-sensitive adhesive as claimed in claim 10 in whichthe first monomer is selected from the group consisting of 2-ethylhexylacrylate, isooctyl acrylate, and mixtures thereof; and the secondmonomer comprises acrylic acid.
 12. A cured pressure-sensitive adhesiveas claimed in claim 8 in which the electron-beam dosage was from about 5to about 50 kGy.
 13. A pressure-sensitive adhesive as claimed in claim12 in which the unsaturated carboxylic is acrylic acid.
 14. A curedpressure-sensitive adhesive which comprises a branched chain acrylicemulsion polymer having a glass transition temperature of from about 20°C. to about 70° C. below the use temperature and having a storagemodulus (G') of from about 3.2×10⁴ Pa to about 1.2×10⁵ Pa at a frequencyof 10⁻¹ s⁻¹ and from about 1.4×10⁵ Pa to about 5.2×10⁵ Pa at a frequencyof 10² s¹ and a loss modulus (G") of from about 9.5×10³ Pa to about3.1×10⁴ Pa at a frequency of 10^(s) and about 7×10⁴ Pa to about 5.6×10⁵Pa at a frequency of 10² s¹, said branched chain acrylic emulsionpolymer formed of about 50 to about 95% by weight of a first monomerwhich when homopolymerized has a glass transition temperature greaterthan -25° C. and at least one second monomer which when homopolymerizedhas a glass transition temperature less than -25° C., said acrylicemulsion polymer being cured in the presence of about 0.2 to about 10%by weight of a multifunctional additive based on the weight of theemulsion polymer and multifunctional additive and exposure to electronbeam radiation at a dosage up to 10 kGy, said electron-beam cured,pressure-sensitive adhesive providing a 180° peel of at least about 250N/m, and a loop tack of at least about 200 N/m and a 70° C. shear of atleast about 10 kiloseconds at a coat weight of 40-60 g/m².
 15. A curedpressure-sensitive adhesive as claimed in claim 14 in which themultifunctional additive is present in a concentration of from about 0.2to about 5% by weight of the acrylic emulsion polymer.
 16. A curedpressure-sensitive adhesive as claimed in claim 14 in which themultifunctional additive is selected from the group consisting ofpentaerythritol triacrylate, trimethylolpropane trimethacrylate, and1,6-hexanediol diacrylate, and mixtures thereof.
 17. A curedpressure-sensitive adhesive as claimed in claim 14 in which the firstmonomer is selected from the group consisting of alkyl acrylates,monoesters of unsaturated dicarboxylic acids, diesters of unsaturateddicarboxylic acids and mixtures thereof.
 18. A pressure-sensitiveadhesive as claimed in claim 17 in which the first monomer is selectedfrom the group consisting of 2-ethyl hexylacrylate, isooctyl acrylate,and mixtures thereof; and the second monomer comprises an unsaturatedcarboxylic acid.
 19. A cured pressure-sensitive adhesive as claimed inclaim 17 in which the second monomer includes an unsaturated carboxylicacid.
 20. A cured pressure-sensitive adhesive as claimed in claim 14 inwhich the 70° C. shear is at least about 86 kiloseconds.
 21. A curedpressure-sensitive adhesive as claimed in claim 14 in which theelectron-beam dosage was from about 5 to about 50 kGy.
 22. A curedpressure-sensitive adhesive comprising an electron-beam cured branchedemulsion polymer having a glass transition temperature of at least 20°C. below the use temperature and formed by emulsion polymerization of atleast one first monomer which, when homopolymerized, has a glasstransition temperature less than -25° C. and at least one second monomerwhich, when homopolymerized, has a glass transition temperature greaterthan -25° C., said branched emulsion polymer cured by exposed toelectron-beam radiation at a dosage of up to about 100 kGy in thepresence of a multifunctional additive present in an positive amount offrom about 0.2 to about 10% by weight of the branched emulsion polymer,the cure providing a 70° C. shear for the branched emulsion polymer ofat least about 10 kiloseconds while providing a 180° peel at least about250 N/m and loop tack of at least about 200 N/m at an adhesive coatweight of from about 40 to about 60 g/m².
 23. A cured-electron beampressure-sensitive adhesive composition as claimed in claim 22 in whichthe multifunctional additive is present in a concentration of from about0.2 to about 5% by weight of the emulsion polymer.
 24. A curedpressure-sensitive adhesive composition as claimed in claim 22 in whichthe multifunctional additive is selected from the group consisting ofpentaerythritol triacrylate, trimethylolpropane trimethacrylate, and1,6-hexanediol diacrylate, and mixtures thereof.
 25. A curedpressure-sensitive adhesive composition as claimed in claim 22 in whichthe first monomer is selected from the group consisting of alkylacrylates, monoesters of unsaturated dicarboxylic acids and diesters ofunsaturated dicarboxylic acids, and mixtures thereof.
 26. A curedpressure-sensitive adhesive composition as claimed in claim 25 in whichthe second monomer includes an unsaturated carboxylic acid.
 27. A curedpressure-sensitive adhesive comprising an electron-beam cured branchedemulsion polymer having a glass transition temperature of at least 20°C. below the use temperature, said branched emulsion polymer prepared ata temperature greater than about 30° C. and cured in admixture with amultifunctional additive present in an amount of from about 0.2 to about5% by weight of the branched emulsion polymer, said emulsion polymersformed by emulsion polymerization of at least one first monomer which,when homopolymerized, has a glass transition temperature less than -25°C. and at least one second monomer which, when homopolymerized, has aglass transition temperature greater than -25° C., said branchedemulsion polymer cured by exposure to electron-beam radiation at adosage of up to about 100 kGy and sufficient to provide 70° C. shear ofat least about 10 kiloseconds while providing a 180° peel at least about250 N/m and loop tack at least about 200 N/m at a coat weight from about40 to 60 g/m².
 28. A cured pressure-sensitive adhesive which comprisesan electron-beam cured branched acrylic emulsion polymer having a glasstransition temperature of at least about 20° C. below the usetemperature and having a storage modulus (G') of from about 3.2×10⁴ Pato about 1.2×10⁵ Pa at a frequency of 10¹ s⁻¹ and from about 1.4×10⁵ Pato about 5.2×10⁵ Pa at a frequency of 10² s⁻¹ and a loss modulus (G") offrom about 9.5×10³ Pa to about 3.1×10⁴ Pa at a frequency of 10⁻¹ s⁻¹ andabout 7×10⁴ Pa to about 5.6×10⁵ Pa at a frequency of 10⁻² s⁻¹, saidbranched acrylic emulsion polymer having been cured in the presence offrom about 0.2 to about 5% by weight of a multifunctional additive, saidbranched acrylic emulsion polymer formed by emulsion polymerization ofabout 50 to about 95% by weight of a first monomer which, whenhomopolymerized, has a glass transition temperature less than -25° C.and at least one second monomer which, when homopolymerized, has a glasstransition temperature greater than -25° C., said branched acrylicemulsion polymer having been exposed to electron-beam radiation at adosage up to about 100 kGy and sufficient to provide a 70° C. shear ofat least about 10 kiloseconds while providing a 180° peel of at least250 N/m and a loop tack of at least 200 N/m at a coat weight of from 40to 60 g/m².
 29. A cured pressure-sensitive adhesive composition asclaimed in claim 28 in which the 70° C. shear is at least about 86kiloseconds.
 30. A cured pressure-sensitive adhesive composition asclaimed in claim 28 in which the electron-beam dosage was from about 5to about 50 kGy.
 31. A cured pressure-sensitive adhesive composition asclaimed in claim 28 in which the multifunctional additive is selectedfrom the group consisting of pentaerythritol triacrylate,trimethylolpropane trimethacrylate, and 1,6-hexanediol diacrylate, andmixtures thereof.
 32. A cured pressure-sensitive adhesive composition asclaimed in claim 28 in which the first monomer is selected from thegroup consisting of alkyl acrylates, monoesters of unsaturateddicarboxylic acids and diesters of unsaturated dicarboxylic acids.
 33. Apressure-sensitive adhesive composition as claimed in claim 32 in whichthe first monomer is selected from the group consisting of 2-ethylhexylacrylate, isooctyl acrylate, and mixtures thereof; and the secondmonomer comprises an unsaturated carboxylic acid.
 34. Apressure-sensitive adhesive composition as claimed in claim 33 in whichthe unsaturated carboxylic is an acrylic acid.
 35. A curedpressure-sensitive adhesive which comprises an electron-beam curedbranched acrylic emulsion polymer having a glass transition temperatureof at least 20° C. below the use temperature, a storage modulus (G') offrom about 3.2×10⁴ Pa to about 1.2×10⁵ Pa at a frequency of 10⁻¹ s⁻¹ andfrom about 1.4×10⁵ Pa to about 5.2×10⁵ Pa at a frequency of 10² s⁻¹ anda loss modulus (G") of from about 9,5×10³ Pa to about 3.1×10⁴ Pa at afrequency of 10⁻¹ s⁻¹ and about 7.10⁴ Pa to about 5.6×10⁵ Pa at afrequency of 10² s⁻¹, said branched acrylic emulsion polymer formed of amixture of at least one acrylic emulsion polymer polymerized in emulsionat a temperature of at least about 30° C., each of said emulsionpolymers formed of about 50 to about 95% by weight of a first monomerwhich, when homopolymerized, has a glass transition temperature lessthan -25° C. and comprising at least one alkyl acrylate and at least onesecond monomer which when homopolymerized, has a glass transitiontemperature greater than -25° C. and comprising at least one unsaturatedcarboxylic acid, said branched acrylic emulsion polymer having beenexposed to electron-beam radiation at a dosage level of from about 5 toabout 100 kGy and sufficient to provide a 70° C. shear of at least about10 kiloseconds in the presence of from about 0.2 to about 5% by weightbased on the weight of the branched chain acrylic emulsion polymer of atleast one multifunctional monomer, said cured emulsion polymers having a180° peel of at least about 250 N/m and a loop tack of at least about200 N/m at a coat weight of from 40 to 60 g/m².
 36. A curedpressure-sensitive adhesive curable on exposure to electron-beamradiation to form a pressure-sensitive adhesive providing elevatedtemperature shear properties which comprises branched acrylic emulsionpolymer having a glass transition temperature of at least 20° C. belowthe use temperature and having a storage modulus (G') of from about3.2×10⁴ Pa to about 1.2×10⁵ Pa at a frequency of 10⁻¹ s⁻¹ and from about1.4×10⁵ Pa to about 5.2×10⁵ Pa at a frequency of 10² s⁻¹ Pa and a lossmodulus (G") of from about 9.5×10³ Pa to about 3.1×10⁴ Pa at a frequencyof 10⁻¹ s⁻¹ and about 7×10⁴ Pa to about 5.6×10⁵ Pa at a frequency of 10²s⁻¹ and formed in emulsion at a temperature less than about 20° C., saidacrylic emulsion polymer formed of about 50 to about 95% by weight of afirst monomer which, when homopolymerized, has a glass transitiontemperature less than -25° C. and comprising one alkyl acrylate and atleast one second monomer which, when homopolymerized, has a glasstransition temperature greater than -25° C., and comprising anunsaturated carboxylic acid, said acrylic emulsion polymer having beenexposed to electron-beam radiation at a level of from about 5 to about50 kGy in the presence of a multifunctional additive in a positiveamount up to 10% by weight of the polymer and sufficient to provide a70° C. shear of at least about 10 kiloseconds while providing a 180°peel of at least 250 N/m and a loop tack of at least about 200 N/m at acoat weight of 40 to 60 g/m².
 37. A cured pressure-sensitive adhesive asclaimed in claim 36 in which there was present during cure amultifunctional additive present in an amount of from 0.2 to about 5% byweight based on the weight of the emulsion polymer.
 38. A curedpressure-sensitive adhesive as claimed in claim 36 in which 70° C. shearis at least about 86 kiloseconds.
 39. A cured pressure-sensitiveadhesive as claimed in claim 37 in which the multifunctional additive isselected from the group consisting of pentaerythritol triacrylate,trimethylolpropane trimethacrylate, and 1,6-hexanediol diacrylate, andmixtures thereof.
 40. A cured pressure-sensitive adhesive as claimed inclaim 39 in which the first monomer is selected from the groupconsisting of alkyl acrylates, monoesters of unsaturated dicarboxylicacids and diesters of unsaturated dicarboxylic acids, and mixturesthereof.
 41. A cured pressure-sensitive adhesive as claimed in claim 40in which the unsaturated carboxylic acid is acrylic acid.